Piezoelectric materials convert energy between mechanical energy and electrical energy. A piezoelectric material generates electrical energy when mechanically deformed (e.g. under mechanical stress). On the other hand, a piezoelectric material may mechanically deform in response to electrical energy. For example, a piezoelectric material may vibrate in response to electrical energy, thereby acting as an ultrasonic transducer to generate acoustic waves. Piezoelectric materials may be used in micro-electro-mechanical system (“MEMS”) devices, for example, in fingerprint recognition.